Radio communication apparatus, method and program

1. A radio communication apparatus comprising: a receiving unit configured to receive a first radio signal indicating a state in which transmission by a Space Division Multiple Access (SDMA) scheme is allowed; a first determination unit configured to determine, upon receiving the first radio signal, whether a transmission state is a first state in which transmission by the SDMA scheme is possible, or a second state in which the transmission by the SDMA scheme is impossible; a second determination unit configured to determine whether the first state continues for a first time period; a first transmission unit configured to transmit a radio signal using the SDMA scheme, when the second determination unit determines that the first state continues for the first time period; and a first setting unit configured to set a third time period instead of the first time period, when the second determination unit determines that the first state fails to continue for the first time period, the third time period being obtained by subtracting, from the first time period, a second time period ranging from when the carrier sense is started to when the transmission state is determined to be the second state, wherein when the receiving unit receives the first radio signal after the third time period is set, the second determination unit determines whether the first state continues for the third time period.

2. The apparatus according to claim 1 , wherein when there is no radio signal from another radio communication apparatus, the first determination unit determines that the transmission state is the first state.

3. The apparatus according to claim 1 , wherein when number of radio signals simultaneously transmitted by the SDMA scheme is less than a maximum spatial multiple number set in the SDMA scheme, the first determination unit determines that the transmission state is the first state.

4. The apparatus according to claim 1 , wherein the receiving unit receives information indicating a fourth time period and included in the first radio signal, the fourth time period being a time period in which the transmission by the SDMA scheme is allowed, and further comprising a third determination unit configured to determine that the transmission state has been shifted to the second state when the first state continues for a time period longer than the fourth time period.

5. The apparatus according to claim 1 , further comprising: a fourth determination unit configured to determine whether the transmission state is a third state in which transmission by a non-SDMA scheme is possible, or a fourth state in which transmission by the non-SDMA scheme is impossible; a fifth determination unit configured to determine whether the third state continues for a fifth time period; a second transmission unit configured to transmit a radio signal using a non-SDMA scheme, when the fifth determination unit determines that the third state continues for the fifth time period; a second setting unit configured to set a seventh time period instead of the fifth time period, when the fifth determination unit determines that the third state fails to continue for the fifth time period, the seventh time period being obtained by subtracting, from the fifth time period, a sixth time period ranging from when the carrier sense is started to when the transmission state is determined to be the fourth state, wherein after the seventh time period is set, the fifth determination unit determines whether the third state continues for the seventh time period.

6. A radio communication apparatus comprising: a receiving unit configured to receive a first radio signal indicating a state in which transmission by a Space Division Multiple Access (SDMA) scheme is allowed; a first determination unit configured to determine, upon receiving the first radio signal, whether a transmission state is a first state in which there are no radio signals of arbitrary communication schemes, or a second state in which there is a radio signal of a communication scheme; a second determination unit configured to determine whether the first state continues for a first time period; a first transmission unit configured to transmit a radio signal using the SDMA scheme, when the second determination unit determines that the first state continues for the first time period; and a first setting unit configured to set a third time period instead of the first time period, when the second determination unit determines that the first state fails to continue for the first time period, the third time period being obtained by subtracting, from the first time period, a second time period ranging from when the carrier sense is started to when the transmission state is determined to be the second state, wherein when the receiving unit receives the first radio signal after the third time period is set, the second determination unit determines whether the first state continues for the third time period.

7. The apparatus according to claim 6 , wherein when the first determination unit determines that the transmission state is the second state, the second determination unit determines that the transmission state has been shifted from the state in which the transmission by the SDMA scheme is possible, to a state in which the transmission by the SDMA scheme is impossible.

8. A radio communication apparatus comprising: an estimation unit configured to estimate number of first radio communication apparatuses having requests for transmission and included in second radio communication apparatuses, when the second radio communication apparatuses receive a first radio signal indicating a state in which transmission by a Space Division Multiple Access (SDMA) scheme is allowed; a setting unit configured to set an upper limit of a random number generated by each of the first radio communication apparatuses when each first radio communication apparatus computes a carrier sense time period, the larger the random number, the longer the carrier sense time period, the upper limit being set to cause a product of the number of the first radio communication apparatuses and an inverse number of the upper limit to be smaller than a maximum spatial multiple number determined in the SDMA scheme by a value falling within a threshold value range; and a transmission unit configured to transmit the first radio signal including the upper limit to third radio communication apparatuses belonging to a network and including the first radio communication apparatuses.

9. The apparatus according to claim 8 , wherein the estimation unit estimates the number of the first radio communication apparatuses based on number of the third radio communication apparatuses belonging to the network.

10. A method of using a radio communication apparatus, comprising: receiving a first radio signal indicating a state in which transmission by a Space Division Multiple Access (SDMA) scheme is allowed; determining, upon receiving the first radio signal, whether a transmission state is a first state in which transmission by the SDMA scheme is possible, or a second state in which the transmission by the SDMA scheme is impossible; determining whether the first state continues for a first time period; transmitting a radio signal using the SDMA scheme, when it is determined that the first state continues for the first time period; setting a third time period instead of the first time period, when it is determined that the first state fails to continue for the first time period, the third time period being obtained by subtracting, from the first time period, a second time period ranging from when the carrier sense is started to when the transmission state is determined to be the second state; and determining whether the first state continues for the third time period, when receiving the first radio signal after setting the third time period.

11. A method of using a radio communication apparatus, comprising: receiving a first radio signal indicating a state in which transmission by a Space Division Multiple Access (SDMA) scheme is allowed; determining, upon receiving the first radio signal, whether a transmission state is a first state in which there are no radio signals of arbitrary communication schemes, or a second state in which there is a radio signal of a communication scheme; determining whether the first state continues for a first time period; transmitting a radio signal using the SDMA scheme, when it is determined that the first state continues for the first time period; setting a third time period instead of the first time period, when it is determined that the first state fails to continue for the first time period, the third time period being obtained by subtracting, from the first time period, a second time period ranging from when the carrier sense is started to when the transmission state is determined to be the second state; and determining whether the first state continues for the third time period, when receiving the first radio signal after setting the third time period.

12. A method of using a radio communication apparatus, comprising: estimating number of first radio communication apparatuses having requests for transmission and included in second radio communication apparatuses, when the second radio communication apparatuses receive a first radio signal indicating a state in which transmission by a Space Division Multiple Access (SDMA) scheme is allowed; setting an upper limit of a random number generated by each of the first radio communication apparatuses when each first radio communication apparatus computes a carrier sense time period, the larger the random number, the longer the carrier sense time period, the upper limit being set to cause a product of the number of the first radio communication apparatuses and an inverse number of the upper limit to be smaller than a maximum spatial multiple number determined in the SDMA scheme by a value falling within a threshold value range; and transmitting the first radio signal including the upper limit to third radio communication apparatuses belonging to a network and including the first radio communication apparatuses.